As is well known, turbine engines typically include a rotor and a turbine wheel rotatable about a generally horizontal axis Not infrequently, an annular combustor surrounds the axis and is provided with a plurality of angularly spaced fuel injectors whereby fuel is injected into the combustor to be burned and ultimately directed at the turbine wheel to spin the same. At a location that is usually external of the combustor, a ring-like manifold is utilized as a fuel manifold that interconnects the various fuel injectors.
Because the rotational axis of the compressor and turbine wheel is typically horizontal, the ring-like manifold will be in a vertical plane. This in turn means that the pressure acting on the fuel at the lowermost injectors is greater than the pressure acting on the fuel at the highest injectors as a consequence of gravity acting on the column of fuel within the manifold itself The pressure difference is due to the pressure head created by the vertical column of fuel in the manifold and thus is termed "manifold head".
In many instances, this does not present a problem. However, in turbines of the sort whereat very low fuel flows may be employed as for example, small turbines operating at high altitude, substantial nonuniformity in fuel injection may result. In some cases, it is possible that fuel injection will occur only at the lowermost injectors and not at all at the uppermost ones.
This, in turn, can lead to the development of hot spots within the turbine engine which shortens its life as well as operating inefficiencies because of poor, localized combustion.
In order to overcome the difficulty, it has been proposed to provide each fuel injector or, in some cases, pairs of fuel injectors, with an orifice The orifices then require an increased fuel injecting pressure in order to deliver fuel past the orifice into the combustion chamber and as a consequence, the manifold head pressure at the lower injectors is relatively small compared to the injecting pressure applied to the fuel at all orifices Thus, substantially uniform injection will occur at all injector locations.
The approach is not altogether satisfactory. For one, in order to increase the pressure drop at each fuel injector sufficiently, the orifices must be made to be relatively small. As a consequence, they are prone to clogging. And, of course, when one or more orifices clog, the corresponding fuel injector is blocked and again, the problem of hot spots arises.
In addition, with orifices, the pressure drop across the orifice rises asymptotically in proportion to fuel flow. This in turn means that undesirably high fuel pressures must be utilized to deliver fuel at high flow rates that are desired for some stages of turbine operation.
To avoid these difficulties, in commonly assigned U.S. Pat. No. 4,862,693 issued Sep. 5, 1989 to Batakis et al, the use of capillary tubes is proposed. While the means therein disclosed do solve the above problem, in some instances an even greater degree of uniformity of injection may be desired
The present invention is directed to providing the "fine tuning" that achieves a high degree of uniformity of injection.